The invention relates to diagnostic imaging, and more particularly relates to magnetic resonance (MR) imaging. In its most immediate sense, the invention relates to MR pulse sequences.
The above-referenced parent patent application, which relates to two-dimensional MR pulse sequences, describes a problem inherent in conventional MR pulse sequences of the GRASE type. This problem is that because of the effect of T2 decay combined with the order in which lines of MR data are acquired, the data in the k-space matrix is highly amplitude-modulated ("AM"). When the k-space data is Fourier-transformed, this amplitude modulation is decoded as multiple edges of diminishing contrast and regular spacing ("ringing artifacts"). These AM ringing artifacts are distracting and interfere with the diagnostic quality of the final MR image.
Conventional three-dimensional MR pulse sequences of the GRASE type also inherently suffer from this problem. This is because a conventional three-dimensional GRASE-type MR pulse sequence is merely a set of two-dimensional GRASE-type pulse sequences, wherein each member of the set acquires all in-plane data at a constant phase-encoding gradient in the Z direction. As a result, conventional three-dimensional MR pulse sequences of the GRASE type inherently suffer from amplitude modulation of the MR data, and inherently produce ghost artifacts, especially when the sequences are long and T2 decay thereby causes a pronounced effect.
One object of the invention is to provide a three-dimensional GRASE-type MR pulse sequence which does not suffer from artifacts caused by amplitude modulation of the acquired MR data as a result of T2 decay.
Another object of the invention is to provide such an MR pulse sequence in which AM artifacts and phase modulation artifacts are completely absent.
Yet a further object of the invention is to improve on MR sequences of this general type.
The invention proceeds from a realization that a three-dimensional MR pulse sequence is subject to less stringent constraints than is a two-dimensional MR pulse sequence. The above-referenced parent patent application discloses a two-dimensional GRASE-type MR pulse sequence in which amplitude modulation of the acquired MR data is traded against phase modulation of the acquired MR data. This tradeoff is necessary because in the there-disclosed MR pulse sequence, phase-encoding takes place exclusively along a single in-plane axis. However, in a three-dimensional MR pulse sequence, phase encoding takes place in two directions, namely along the in-plane axis and the Z-axis. As a result, amplitude modulation need not be traded off against phase modulation. Indeed, it is possible to acquire lines of MR data using a scheme of GRASE-type phase encoding wherein artifacts resulting from amplitude modulation and from phase modulation are completely eliminated.
In accordance with the invention, there is provided a three-dimensional GRASE-type MR pulse sequence wherein, between each two adjacent RF refocussing pulses, the phase-encoding along the Z axis is varied identically. There consequently is no in-plane amplitude modulation of the acquired MR data. So, too, there consequently is no in-plane phase modulation of the acquired MR data. Advantageously, and in accordance with the preferred embodiment, the in-plane phase encoding remains constant between each two adjacent RF refocussing pulses and varies monotonically from the beginning of the MR pulse sequence to the end.